The present invention relates to a lightning surge protector (LSP) for protecting power transmission/distribution equipment from an abnormal voltage caused by a lightning surge.
FIGS. 1 and 2 are longitudinal sectional views each illustrating a basic structure of the conventional LSP having a non-linear resistive current limiting element (hereinafter simply referred to as "current limiting element").
The LSP shown in FIG. 1 comprises electrode members such as an upper electrode plate 103, a lower electrode plate 104 and a spring 109 which are housed and fixed in an inner space defined by a cylindrical pressure-proof insulating housing 111 such as FRP. An upper electrode metal member 105 and a lower electrode metal member 106 are coupled with the upper and lower ends of the housing by means of screws. The outer wall surface of the pressure-proof housing 111 is covered with an insulating coating 107 of an organic insulating material. The inner space of the housing is filled with an organic insulating material 108.
The LSP shown in FIG. 2 is similar to the above-mentioned LSP, only the pressure-proof insulating housing 111 is replaced by an insulator 112, and electrode members such as the upper electrode plate 103, the lower electrode plate 104 and the spring 109 are housed and fixed in the inner space defined by the insulator 112. Also, the upper electrode metal member 105 and the lower electrode metal member 106 are coupled with the upper and lower ends of the housing by means of screws, and the inner space portion is filled with an insulating gas 113.
A pressure-release structure is provided in each of these basic structures as a counter-measure for safety in the case of a failure of the surge protecting device.
Furthermore, LSPs having a current limiting element are disclosed in Japanese Unexamined Patent Publication Nos. Sho-61-151913 and Sho-60-70702.
FIG. 3 is a longitudinal sectional view illustrating an LSP of the former Publication, comprising an arcing ring 226 attached to a structure in which a current limiting element 222 is housed in a pressure-proof insulating cylinder 221. The insulating cylinder has pressure-release holes 224 formed in its side surface, and the outside and inside of the pressure-proof insulating cylinder 221 are covered and filled with an insulating material 223. The reference numeral 225 designates an electrode.
FIG. 4 is a longitudinal sectional view illustrating an LSP of the latter Publication, in which a current limiting element 232 is housed in a pressure-proof insulating cylinder 231. Pressure-release valves 233 and pressure-release openings 234 are provided in each of the upper and lower portions of the cylinder 231.
In each of the above-mentioned conventional LSPs, in the case of an ordinary lightning surge, the surge is passed by the current limiting element and the insulating state is recovered in the condition of a transmission voltage to thereby prevent a service interruption. On the contrary, the case where a penetrating-shorting fault or a creeping-flashover fault occurs in the current limiting element by a lightning surge exceedingly larger than a designed valve, an arc of high temperature and high pressure is produced inside of the pressure-proof insulating cylinder so that the LSP explodes and flies about.
In order to prevent this, in the LSP of FIG. 3, the organic insulating material over a pressure-release hole is broken through by the arc pressure in the initial stage of a flashover. In the LSP of FIG. 4, on the other hand, the upper and lower pressure-release valves are opened by the arc pressure to discharge an arc jet, and a gas ionized by the arc energy is blown to the outside arcing horns so as to change the course of the arc from the inside of the LSP to the outside to prevent the LSP from exploding and flying about.
FIG. 5A is a diagram illustrating an example of the LSP for a transmission line. FIG. 5A depicts a steel tower 251, an overhead earth wire 252, a transmission line 126, an LSP 124, an insulator 122 and a series gap 127. FIG. 5B is an explanatory diagram showing an example of the application of an LSP, and FIG. 5C is a circuit diagram illustrating an LSP apparatus.
An overhead transmission/distribution line 126 is suspended from a support steel crossarm 121 of a steel tower by a support insulator 122. Arcing horns 123 are attached to the upper and lower ends of the support insulator 122. An LSP 124 is disposed in parallel to the support insulator 122, and a series gap 127 is provided between the lower end portion of the support insulator 122 and the lower end portion of the LSP 124. The distance of the series gap 127 is less than the distance of the arcing horn gap and larger than the maximum arcing distance of the switching surge flashover voltage.
In normal operation of the thus arranged LSP apparatus, if an electric shock 128 is given to the steel tower, the voltage across the support steel crossarm 121 and the transmission/distribution line 126 becomes high suddenly. However, a flashover will occur across the series gap 127 before a flashover between arcing horns 123 so that a lightning surge current flows through the LSP 124. At the transmission voltage after the lightning surge voltage, insulation is recovered by the characteristic of a current limiting element included in the LSP 124 to thereby prevent service interruption.
Thus, in order to make the series gap 127 flashover so quickly that the gap of the arcing horns 123 of the support insulator 122 cannot flashover when a lightning surge voltage V.sub.1 is applied, the potential gradient V.sub.2 (V/cm) across the series gap 127 must be higher than the potential gradient V.sub.3 (V/cm) across the arcing horns 123. The share voltage ratio of the LSP 124 to the series gap 127 upon application of a lightning surge voltage is determined by the electrostatic capacity ratio of the electrostatic capacity C.sub.1 of the LSP 124 to the electrostatic capacity C.sub.2 of the series gap 127.
However, in the case of the above-mentioned conventional LSP, the upper and lower electrode members are connected to each other by an insulating material. The electrostatic capacity C.sub.1 of the arrester becomes small as seen in the equivalent circuit shown in FIG. 6A, so that the ratio of the electrostatic capacity C1 to the electrostatic capacity C2 of the series gap becomes .perspectiveto.1. The potential gradients of V.sub.2 and V.sub.3 are therefore close to each other, so that there is a possibility that the arcing horns 123 on the support insulator 122 side will flashover. It is therefore necessary to make a change such as enlarging the distance between the arcing horns 123 on the support insulator 122 side. In FIG. 6A, C.sub.01 to C.sub.05 represent respective electrostatic capacities of current limiting elements, and C.sub.11 represents an extremely small electrostatic capacity across the upper and lower electrode members.
Furthermore, since each of the above-mentioned conventional LSPs is constituted by a current limiting element, a pressure-proof insulating cylinder, pressure-release apertures or valves, and an arcing ring or horns, there exist the following problems:
(i) since each LSP is not of an arc-extinguishable structure, generation of arc energy continues even while a shorting current flows, so that there is a potential for fire; PA1 (ii) if pressure-release holes or valves are blocked by broken pieces of the current limiting element or the like, the blow off of an arc jet may be delayed, possibly damaging the pressure-proof insulating cylinder; PA1 (iii) a harmful gas at high temperature and high pressure is produced and exhausted into the air; PA1 (iv) there is a fear that a part of the structure may fly about; causing damage or injury and PA1 (v) arcing rings or arcing horns and a pressure-release mechanism are necessary, thus complicating the structure.